Python 1 Home

Regular Expressions

Regular expressions (regex or regexp) are powerful tools for pattern matching within strings. They are incredibly useful for tasks like data validation, text manipulation, and searching.

Key Concepts:

• Metacharacters: Special characters that have specific meanings in regex (e.g., . for any character, * for zero or more repetitions, + for one or more repetitions, ? for zero or one repetition, 1 for character sets, () for grouping, ^ for beginning of string, $ for end of string). Regex Metacharacters
• Quantifiers: Specify how many times a character or group should appear (e.g., *, +, ?, {n}, {n,}, {n,m}). Regex Quantifiers
• Anchors: Match positions within a string, not characters (e.g., ^, $).
• Character Sets: Define a set of characters to match (e.g., [abc]], [a-z]], [^abc]]). Regex Character Sets
• Grouping and Capturing: Use parentheses () to group parts of a regex and capture matched substrings. Regex Grouping and Capturing
• Alternation: Use the pipe symbol | to specify alternative patterns.
• Flags: Modify the behavior of the regex engine (e.g., re.IGNORECASE for case-insensitive matching). Regex Flags

Python’s re module provides functions for working with regular expressions:

import re

text = "My phone number is 123-456-7890 and email is test@example.com"

# Find phone number
phone_number_match = re.search(r"\d{3}-\d{3}-\d{4}", text)
if phone_number_match:
    print(f"Phone number: {phone_number_match.group(0)}")

# Find all email addresses (this assumes a simple email format)
email_matches = re.findall(r"[a-zA-Z0-9._%+-]]+@[a-zA-Z0-9.-]]+\.[a-zA-Z]]{2,}", text)
print(f"Email addresses: {email_matches}")

# Replace phone number with ***-***-****
new_text = re.sub(r"\d{3}-\d{3}-\d{4}", "***-***-****", text)
print(f"New text: {new_text}")

Related Notes:

• Regex Groups and Backreferences

Further Exploration:

• Practice writing regexes for different scenarios. Online regex testers can be helpful.
• Learn about more advanced techniques like lookahead and lookbehind assertions.